1. Field of the Invention
The present invention relates to a thin film transistor, and in particular to a buffer layer within the thin film transistor for promoting electron mobility.
2. Description of the Related Art
Thin film transistor (TFT) is of the conventional LCD driver. According to the materials of active layer of TFT, the TFT can be divided into amorphous silicon TFT (a-Si:H TFT) and a polysilicon TFT. The polysilicon can also be divided into high temperature (HTPS) and low temperature polysilicon (LTPS) according to the manufacturing process.
In conventional low temperature polysilicon process, amorphous silicon is heated by excimer laser annealing (ELA) to recrystallize to form polysilicon. In order to increase adhesion, a buffer layer is usually deposited between the amorphous silicon and the substrate. The buffer layer can also serve as a block to prevent particles from diffusing into the active. The conventional buffer layer is usually silicon oxide of about 3000 Å.
Manufacture of a thick silicon oxide is time-consuming and costly. A bi-layer buffer layer comprising silicon oxide and a silicon nitride has thus been developed.
However, disadvantages of the bi-layer buffer layer were proposed by Naoya et al in the Journal of Active-Matrix Liquid-Crystal Display-TFT in 2002. Usually, hydrogen is produced by silicon nitride during manufacture. During excimer laser annealing (ELA), the hydrogen contained in the silicon nitride buffer layer diffuses into the active layer via silicon oxide buffer layer, such that stress is introduced by the hydrogen diffusion, causing limitation of grain growth. As a result, electron mobility in the active is reduced.